This invention relates generally to thermal resistors and more specifically to applications for positive temperature coefficient (PTC) resistors.
PTC resistors have been utilized within telephone line interface circuits as disclosed within U.S. Pat. No. 4,467,310 entitled xe2x80x9cTelephone Subscriber Line Battery Feed Resistor Arrangementsxe2x80x9d by Jakab and assigned to the assignee of the present invention, herein incorporated by reference. The use of such PTC resistors within patent ""310 is done to reduce the thermal risks in the event that a relatively high external voltage is applied to one or both of the wires of the subscriber line pair. This situation could occur if a power line came into contact with the subscriber line. In such a circumstance, the resulting high current flow could cause the battery feed resistors within the applicable line interface circuits to heat up, further causing semiconductor devices within the line interface module to exceed their maximum rated junction temperatures.
To prevent this risk, PTC resistors, as disclosed within the ""310 patent, are placed in series with thick film resistors that together with the PTC resistors function as the battery feed resistors. In normal operation, the thick film resistors dominate the total impedance of each battery feed resistor, since the thick film resistors have impedances in the order of ten to thirty times those of the PTC resistors. During a period of extremely high current, the PTC resistors increase in temperature, which causes a drastic increase in their impedances and therefore a drastic increase in the impedances of the corresponding battery feed resistors. This limits the current flow, which subsequently avoids any further rise in temperature. After the high current source has been removed from the particular telephone lines, the impedance of the PTC resistors drops back to normal levels as the temperature is reduced.
One key problem with such a set up is the fact that although the impedances of the PTC resistors in normal operation are small relative to the values of their corresponding thick film resistors, the PTC resistors still significantly influence the overall impedance of the battery feed resistors and the critical balancing within each pair of subscriber lines. As disclosed within the ""310 patent, the battery feed resistors should be matched within 0.3% to maintain good balance within the pair of subscriber lines in which the resistors are attached. Unfortunately, PTC resistors are not extremely accurate. The PTC resistors, within the setup disclosed in the ""310 patent, are approximately 3% to 10% of the overall impedance of the battery feed resistors during normal operations and are only accurate to approximately 25%.
The ""310 patent solves this matching problem by utilizing two PTC resistors manufactured in the same batch, so that the PTC resistors presumably would be more closely matched. Although the use of PTC resistors from the same batch reduces the problem, it does not eliminate it in all circumstances. After a PTC resistor is driven to a high impedance state, during a high temperature period, the PTC resistor does not necessarily return to precisely the same impedance as it had been prior to the high impedance period. In such cases, it is unlikely that the PTC resistors continue to be well matched, and therefore unlikely that the overall battery feed resistors are matched within the acceptable error range of 0.3% between the individual subscriber lines. Due to these problems which reduce the performance of the telephone system as a whole, the addition of PTC resistors within line interface circuits have not been commonly implemented.
Prior art line interface circuits have been implemented with alternative safety features that function to combat similar high voltage problems as set out herein above. The most common safety feature utilized within line interface circuits is electrical relays. These relays are placed between the line interface circuit and the telephone lines. In the situation that the voltage on the telephone lines exceeds a threshold, the relays automatically cause an open circuit of the line interface circuit, therefore preventing damage to the particular interface circuit. Unfortunately, relays take up significant space on a line interface circuit, are not economical in all situations, and require power supplied by the telephone central office to operate properly.
Alternatively, some prior art line interface circuits include fuses between the line interface circuits and the telephone lines, in order to prevent high voltages on the telephone lines from causing damage. These fuses are made to blow, and therefore leave an open circuit, when the voltage on the subscriber lines exceeds a threshold, similar to a relay. The advantages of using fuses over relays is that fuses are smaller, cheaper, and operate without requiring power from the central telephone office. The key disadvantage of fuses over both the electrical relays and the PTC resistor implementation, disclosed within the ""310 patent, is that once a fuse is blown, there is no method for resetting it. Therefore, if a fuse blows, the entire line interface module in which the line interface circuit is implemented must be replaced. Since a very common technique in line interface modules is to implement 32 per board, if a fuse is blown in one circuit, the entire board must be replaced, hence increasing financial and time costs.
Overall, the implementation of PTC resistors appears to have the best qualities of both the electrical relays and the fuses. PTC resistors within a line interface circuit are economical, do not require power to operate properly, and automatically reset after the high voltage source is removed from the telephone line. The key disadvantage of PTC resistors, as presently implemented, is the lack of accuracy with regard to their impedances, especially after returning from a high impedance state.
Hence, an implementation of PTC resistors is needed that will allow the functionality of PTC resistors to be realized, without affecting the overall performance of an impedance sensitive circuit. This implementation should be applicable within a telephone line interface circuit, such that the PTC resistors can realize similar benefits as those described in the ""310 patent, without causing additional problems within the overall circuitry.
It is an object of the present invention to overcome the disadvantages of the prior art and, in particular, to provide an apparatus whereby a PTC resistor can be efficiently utilized.
According to a first aspect, the present invention provides an apparatus comprising: at least one first node; at least one first resistor that increases in temperature under predetermined conditions; at least one positive temperature coefficient (PTC) resistor, thermally coupled and electrically coupled in series to the first resistor, that significantly increases in impedance when the first resistor and hence the PTC resistor significantly increases in temperature; a current detection device, coupled to the first node that detects the current flowing through the first node to generate a detected current signal; a voltage detection device, coupled to the first node, that detects the voltage on first node to generate a detected voltage signal; and an impedance adjustment device, input with the detected current and voltage signals, that, in normal operation, adjusts the current flowing through the first node and the voltage on the first node to maintain approximately a predetermined desired impedance level on the first node; wherein at least one of the first resistor and the PTC resistor is coupled to the first node.
According to a second aspect, the present invention provides an apparatus arranged to be coupled to a first and second transmission line at first and second nodes respectively comprising: first and second resistors that increase in temperature under predetermined conditions; first and second positive temperature coefficient (PTC) resistors, thermally coupled and electrically coupled in series to the first and second resistors via third and fourth nodes respectively, that significantly increase in impedance when the first and second resistors and hence the first and second PTC resistors respectively significantly increase in temperature; a current detection device, coupled to the first and second nodes, that detects a difference in the current flowing through the first and second nodes to generate a differential current signal; a voltage detection device, coupled to the first and second nodes, that detects a difference in voltage on the first and second nodes to generate a differential voltage signal; and an impedance adjustment device, input with the differential current and voltage signals, that, in normal operation, adjusts the current flowing through the first and second nodes and the voltage on the first and second nodes to maintain approximately a predetermined desired impedance level between the first and second nodes; wherein at least one of the first resistor and first PTC resistor is coupled to the first node and at least one of the second resistor and the second PTC resistor is coupled to the second node.
According to a third aspect, the present invention provides a line interface circuit for supplying energizing current from power terminals to a two wire communication line and for coupling communications signals between the communication line and a telephone facility via a hybrid circuit means, comprising: tip and ring terminals for connection to the two wire communication line; hybrid transmit and receive terminals for connection to the hybrid circuit means; a tip and ring signal voltage detector, being responsive to differential signals appearing across the tip and ring terminals and being of at least a voice band frequency, and being responsive to signals appearing at the receive terminal, for generating a composite signal; a loop driver circuit including a voltage amplifier having an input for receiving a control signal and an output, and a transformer having a tip winding being coupled in series with the tip terminal and one of the power terminals, a ring winding coupled in series between the ring terminal and another of the power terminals, the tip and ring windings providing paths for the energizing current to flow and being poled such that energizing current flow is of a flux aiding effect, and a primary winding being coupled in series with the output of the voltage amplifier; the loop driver circuit being responsive to the control signal for driving alternating current signals via the tip and ring terminals; a loop current detector, being coupled in series between the tip and ring terminals and the loop driver circuit, for generating a supervision signal in response to current flow in the communication line; a network having a first port being connected to receive the composite signal from the tip and ring signal voltage detector, a second port being connected to receive the supervision signal from the loop current detector, and a third port being connected to the input of the voltage amplifier for providing the control signal; a tip positive temperature coefficient (PTC) resistor coupled between the tip terminal and the tip winding; and a ring PTC resistor coupled between the ring terminal and the ring winding; wherein the tip and ring PTC resistors are each thermally coupled to at least one resistor coupled to the tip and ring terminals respectively; and each particular PTC resistor will significantly increase in impedance, if the particular resistor that the PTC resistor is thermally coupled to significantly increases in temperature, hence increasing the temperature of the particular PTC resistor.